Polystyrene (PS) is one of the largest volume thermoplastic resins in commercial production today. This ubiquitous material is well suited to many "low performance" applications wherein its brittle nature is of little consequence. Additionally, many other applications requiring greater impact resistance have been uncovered by the advent of various modifications of these plastics. Thus, styrene-based copolymers, and particularly PS resins which are modified with organic rubber particles, have been a commercially viable alternative to some of the more exotic and expensive engineering plastics for certain applications.
One such system, known in the art as high impact polystyrene (HIPS), can have an impact strength which is an order of magnitude greater than the virgin resin but suffers from poor thermal stability, particularly in the presence of oxygen. These modified PS resins are typically prepared by polymerizing a solution of an unsaturated organic rubber, such as polybutadiene, in styrene monomer.
The addition of various rubber compositions to other thermoplastic resin systems has also proved beneficial. For example, Japanese Kokai Patent Application No. Hei 2(1990)-263861 to Mitsubishi Rayon Co., Ltd. discloses a thermoplastic resin composition having a high impact strength, high heat resistance and good resistance to organic solvents. This composition comprises a blend of polyphenylene ether (PPE) resin, a polyester resin and a rubber-like elastomer and/or modified rubber-like elastomer. A preferred elastomer component of this prior art disclosure is obtained by the graft copolymerization of at least one vinyl monomer with a composite rubber consisting of a silicone rubber and a polyalkyl methacrylate interlocked with each other. In the production of the composite rubber component, a cyclic diorganosiloxane is emulsion polymerized with a crosslinker and, optionally, with a graft crosslinking agent using a sulfonic-acid-series emulsifying agent. In a subsequent step, a combination of an alkyl (meth)acrylate, a crosslinker and a graft crosslinking agent is used to swell the silicone particles of the above emulsion and an initiator is then introduced to polymerize this system.
In a similar approach, Alsamarraie et al. U.S. Pat. No. 5,047,472 teaches thermoplastic molding compositions comprising PPE resin, or a PPE resin containing a polystyrene resin, which is modified with a multi-stage polyorganosiloxane/vinyl-based graft polymer. These compositions are stated to have improved impact resistance, flame resistance and moldability. In this case, the graft copolymer is prepared by a "co-homopolymerization" technique wherein an emulsion containing a diorganosiloxane, a crosslinker and a graft-linker is polymerized concurrently with the polymerization of a vinyl monomer. The resulting first stage polymeric co-homopolymerized substrate is then grafted with a vinyl polymer in at least one subsequent stage. This multi-stage polydiorganosiloxane polyorganosiloxane/vinyl-based graft polymer formed according to the methods described by Alsamarraie et al. was also employed by Derudder et al. in U.S. Pat. No. 4,939,205 to augment the impact resistance of polycarbonate resin compositions. The graft polymer was further used by Wang in U.S. Pat. No. 4,939,206 to modify various thermoplastic resins with the object of providing flame retardant compositions having improved impact resistance.
Sasaki et al. U.S. Pat. No. 4,690,986 teaches a polydiorganosiloxane/polystyrene copolymer. However, Sasaki et al. is distinguishable from the present invention in several significant respects. Sasaki teaches that crosslinking occurs between the polydiorganosiloxane molecules during an emulsion polymerization. The Sasaki method teaches that the grafting polymer attaches to the polydiorganosiloxane during the emulsion polymerization at the same time as the formation of crosslinks between the polydiorganosiloxane molecules. Further, the crosslinking and attachment of the grafting polymer occurs randomly in Sasaki. As a result, neither the molecular weight nor the crosslinking density of the polydiorganosiloxane can be controlled. The resulting polydiorganosiloxane is a crumb rubber which is difficult to process during the copolymerization steps. Crumb rubbers are virtually impossible to characterize, for example, in terms of measuring their molecular weights.
In contrast, the present invention teaches that the grafting polymer is attached to an end-capped polydiorganosiloxane during a condensation reaction in the presence of a tin catalyst. The attachment of the grafting group does not occur randomly on the polydiorganosiloxane during an emulsion polymerization, but instead attaches specifically at the two the end-groups of the polydiorganosiloxane molecules. Because it is not crosslinked, the polydiorganosiloxane of the present invention is in the form of a gum and not an elastomer upon removal of water from the emulsion. The gum is easy to characterize, i.e., the molecular weight of the gum can be readily measured by gel permeation chromatography. Upon copolymerizing the telechelic polydiorganosiloxane emulsion with styrene monomer, and removing the water, the composition of the present invention has the consistency of a crumb rubber.